Short mode names in status line.
Add red accent to modified flag, augmenting the default.

Waiting on bling/vim-airline#316 to see if there is a better way to do this.
Better way of adding modified flag to status line.

See https://github.com/bling/vim-airline/issues/316 for more info.
Don't disable existing modification detection; just add red '+'.
Add gitgutter extension and config.
Remove numbers plugin.
No need for actual line numbers; it's in the status line ':<line-number>' works.